I could understand a washout based on MN, but I don't get why it would start at M.3 : that's hardly 200KTAS at SL if I'm not too rusty, so you'd get there pretty quickly
So if the 744 washout is tied to altitude (which makes sense), the Mach-driven washout I read about must be about something else. What I mean is there must be a reason why it is happening so soon, and I'd love to hear about it :-)
Cheers
--
Romain